1. Field of the Invention
The present invention relates to a heat resistance improver which is considerably resistant to heat and light applied during repetitive use outdoor and which is used in a reversible thermosensitive recording medium, and to a reversible thermosensitive recording medium.
2. Description of the Related Art
Conventionally known thermosensitive recording media employ color development reaction between an electron-donating color-developing compound (hereinafter may be referred to as a “color former” or “leuco dye”) and an electron-accepting compound (hereinafter may be referred to as a “color developer”). With the development of office automation, the thermosensitive recording media have widely been used as output sheets for use in facsimiles, word processors and scientific measurement instruments. In recent years, they have also been used as magnetic thermosensitive cards such as prepaid cards and point cards. Thus, in view of environmental problems and recycling, a reversible thermosensitive recording medium capable of being overwritten as often as desired has been developed.
In order for the thermosensitive recording media to exhibit improved light resistance, an ultraviolet (UV) ray-cutting agent is incorporated into a layer constituting them. Examples of known ultraviolet (UV) ray-cutting agents include UV-reflecting agents and UV absorbers. Examples of the UV-reflecting agent include metal oxides such as zinc oxide. Examples of the UV absorbers include benzotriazole (BTA)-based UV absorbers, benzophenone (BP)-based UV absorbers, salicylic acid (SA)-based UV absorbers and cyanoacetic acid (CA)-based UV absorbers. Among them, BTA-based UV absorbers and BP-based UV absorbers are preferred, since their light absorption wavelength is advantageous from the viewpoint of improving light resistance of the media.
The reversible thermosensitive recording media are very advantageous in that they can be repeatedly used. However, in the reversible thermosensitive recording media containing the BTA-based or BP-based UV absorber, the UV absorber (UVA) is bled out after repetitive use, resulting in that the effect thereof cannot be attained for a long period of time.
In order to prevent the absorber from bleeding out, Japanese Patent Application Laid-Open (JP-A) No. 2004-276410 proposes use of a polymer formed from the UVA. Also, Japanese Patent (JP-B) No. 3781587 proposes that a reactive residue is introduced into the UVA itself, and the thus-obtained UVA is reacted with a crosslinking agent in the matrix for immobilization.
These can sufficiently prevent the absorber from bleeding out when the recording medium is used indoor under artificial light.
In recent years, reversible thermosensitive recording media have been increasingly required to be used as a visually recognized medium for RF-ID information, and have been increasingly used outdoor under natural sunlight in, for example, material management in logistics. The natural light is higher in UV dose than the artificial light. Thus, the above-described reversible thermosensitive recording media are not practically applicable outdoor, since they have insufficient UV absorbability.
Also, in addition to the above techniques of UV cutting, JP-A Nos. 2006-88445 and 2006-82252 disclose that a reversible thermosensitive recording medium is prevented from oxidation so as to exhibit improved light resistance. Specifically, these propose provision of a layer which contains a resin capable of forming a hydrogen bond and preventing permeation of oxygen. The formed reversible thermosensitive recording medium exhibits a certain light resistance under low humidity conditions, but exhibits no light resistance under high humidity conditions since the resin used has water solubility and water absorbability.
Also, in order for a UV absorber itself to exhibit enhanced UV absorbability, the UV absorber has been improved so as to absorb light having a longer wavelength. Such UV absorbers that absorb light having a longer wavelength are disclosed in, for example, JP-A No. 2003-521550, Japanese Patent Application Publication (JP-B) Nos. 06-53733 and 07-88471, and JP-A Nos. 09-34057, 10-140089, 05-150433 and 2007-138184.
Reversible thermosensitive recording media containing a UV absorber disclosed in the above literatures involve fogging caused by an erase bar, do not have surfaces resistant to repetitive use outdoor, and do not have sufficient light resistance and heat resistance with respect to formed images and images after erasure. Thus, demand has arisen for development of further improved reversible thermosensitive recording media.